Help with Mauser 1871/84 unit ID

[dksck]

I have a friend who has a Mauser 1871/84 with a unit mark that I promised to identify for him. I pretty sure of the the unit, but since I am very much an amateur at this, I was hoping that others could identify the mark. Unfortunately, I have not been able to obtain a photo. The mark on the the top tang of the buttplate is:

G.R.J. 3.109

The rifle has no other unit marks, nothing has been crossed out or cancelled and there are are no import marks. Thanks for your time and attention, but most importantly, thank you for your knowledge and support.

[The Prussian]

Hello!

The marking stands for Garde-Reserve-Jäger-Bataillon, 3rd company, weapon 109

[dksck]

Thank you for the information. I was thinking Garde Reseve, but I really wasn't certain what specific unit of the Garde Reserve. I am still a little confused. In looking at various sources, I see references to three different units and don't understand if they are all the same unit just called something else or are they different? Could you explain the differences so that I can understand? What is the difference among:
Garde-Reserve Infanterie Regiment Nr. 1

Garde-Reserve Jager Batallion

Garde-Reserve Schutzen Batallion

Thank you for all of your help.

[The Prussian]

Hello!

All of those units were different units! Each unit (GRI, GRJ, GRS) had their own reserve units.

In the war most of the kind of units had to do infantry-work. Jäger were used to fight in woods, mountains and they had bicycle-units. Schützen are a kind of Jäger, but the main difference is just the word, because Schützen had a special tradition